Semiconductor device with protective means against overheating

ABSTRACT

A semiconductor substrate has a power region and a control region. The control region is located in the center portion of the substrate, and the power region surrounds the control region and is separated therefrom. A vertical type, MOS transistor, i.e., an active semiconductor element, is formed on the power region. An insulation film is formed on part of the control region. A polycrystalline silicon diode, which functions as a heat-sensitive element, is formed on the insulation film. A control section comprising a lateral type, MOS transistor is also formed on the control region. The lateral type, MOS transistor is connected to receive a signal form the polycrystalline silicon diode. Further, a polycrystalline silicon resistor, which determines a circuit constant, is formed on the insulation film. The MOS transistor protects the active semiconductor element in response to a signal supplied from the heat-sensitive element showing that the temperature of the semiconductor substrate has risen above a predetermined value. For example, the active semiconductor element may be disabled until the detected temperature drops below a predetermined value.

BACKGROUND OF THE INVENTION

This is a division of application Ser. No. 935,718, filed Nov. 28, 1986,issued on July 26, 1988 as U.S. Pat. No. 4,760,434.

The present invention relates to a semiconductor device with protectivemeans against overheating, and more particularly to a semiconductordevice which has means for protecting the circuit section of the devicefrom overheating due to the excessive temperature rise of thesemiconductor elements provided within the circuit section.

In a semiconductor device comprising a circuit section having activeelements, the junctions of these elements generate heat as the deviceoperates. When the temperature of the junctions rises above a particularvalue, the circuit section will be broken down. A semiconductor deviceis known which is designed to protect itself from overheating. In thisdevice, a heat-sensitive element such as a heat-sensing transistor isformed on a semiconductor substrate, along with the active elements, andthe output signals of the heat-sensitive element control the activeelements.

In the semiconductor device, the active elements and the heat-sensitiveelement are formed on the same semiconductor substrate. It is thereforedifficult to trim the substrate after forming these elements, such thatthe elements can have design precision. Also, since the heat-sensitiveelements are not electrically insulated from that portion of thesubstrate which surrounds the heat-sensitive element, parasitic actionswill occur inevitably between the active elements.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductordevice which has a circuit section and means which can effectivelydetect an excessive temperature rise of the active elements provided inthe circuit section and can protect the circuit section from overheatingdue to an excessive temperature rise of the active elements.

Another object of the invention is to provide a semiconductor devicecomprising a heat-sensitive element for detecting an excessivetemperature rise of the active elements. The heat-sensitive element iselectrically insulated from the active elements. Due to this electricalinsulation, the active elements and the heat-sensitive element canstably operate.

Another object of this invention is to provide a semiconductor devicecomprising a semiconductor substrate, a circuit section including activeelements formed on the substrate, and a heat-detecting section includingheat-sensitive elements formed on the semiconductor substrate fordetecting a temperature rise of the substrate, wherein the circuitsection can be reliably protected from overheating.

Still another object of the invention is to provide a semiconductordevice comprising a semiconductor substrate, and a heat-detectingsection formed on the substrate and composed of PN junctions ofpolycrystalline silicon, whereby the device can withstand a high voltageand a stable temperature characteristic, and thus can be reliablyprotected from overheating.

According to the present invention, there is provided a semiconductordevice comprising a semiconductor substrate, a semiconductor circuitformed on the substrate and including active elements, an insulationfilm formed on at least one portion of the substrate, a heat-sensingelement formed on the insulation film for measuring the temperature ofthat portion of the substrate, and a control section for controlling thesemiconductor circuit in accordance with an output signal from theheat-sensitive element, thereby protecting the circuit from overheating.

As long as the junctions of the active elements undergo no excessivetemperature rises, and the substrate is not excessively heated, thecontrol section does not control the semiconductor circuit to protectthe same from overheating. When the heat-sensitive element detects thatthe temperature of the substrate rises above a predetermined value,however, it generates an output outputted signal. The signal causes thecontrol section to protect the circuit from overheating. More precisely,the control section forcibly stops the semiconductor circuit, therebypreventing the active elements from being broken down by the heatgenerated at their junctions.

As has been stated, an insulation film is formed on the semiconductorsubstrate. This film electrically insulates or separates theheat-sensitive element from the substrate. Therefore, not only can thefilm reliably prevent parasitic actions between the active elements, butit also makes it possible to trim the semiconductor substrate and theheat-sensitive element, independently of each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a semiconductor device according to oneembodiment of the present invention, showing the positions of thevarious regions of the device;

FIG. 2 is a cross-sectional view of the device, taken along line α--α inFIG. 1;

FIG. 3 is an equivalent circuit diagram of the control section providedwithin the semiconductor device;

FIG. 4 is a graph illustrating the relationship between the temperatureof the junction of a power MOS transistor and the gate voltage of thetransistor, and also the relationship between the temperature and thedrain voltage of the transistor;

FIG. 5 is a cross-sectional view of another control section which can beprovided in the device of FIG. 1;

FIGS. 6 to 9 are graphs representing the characteristics of thepolycrystalline silicon diodes used in the device of FIG. 1;

FIG. 10 is a circuit diagram of still another control section:

FIG. 11 is a graph showing the relationship between the gate voltage ofthe MOS transistor used in the control section of FIG. 10 and thetemperature of the junction of the MOS transistor; and

FIG. 12 is a circuit diagram of a further control section which can beused in the device of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematical plan view of a semiconductor d®vice aCCOrding tothe invention, which has means for protecting the device fromoverheating. As shown in FIG. 1, the device comprises semiconductorsubstrate 11. The greater part of substrate 11 is power region 12. Asemiconductor circuit including active elements is formed in powerregion 12. The center portion of substrate 11, which has a lowheat-radiation efficiency and can easily be heated to a hightemperature, is control region 13 which functions to detect temperature.Bonding pads 14 and 15 also are formed on semiconductor substrate 11.Pad 14 is used to draw the gate electrodes of the active elements, andpad 15 is used to draw the source electrodes of the active elements.

FIG. 2 is a cross-sectional view of the semiconductor device of FIG. 1,taken along line α--α. Particularly, the figure shows control region 13and that portion of power region 12 which surrounds region 13. As isshown in FIG. 2, a vertical type power MOS transistor (an activeelement) 22 is formed in power region 12. Other power MOS transistors(not shown) are formed in power region 12 as well. MOS transistor 22 andother power MOS transistors (not shown) are arranged in rows andcolumns, and are connected in parallel, thereby forming a semiconductorcircuit of multi-source structure.

Insulation film 24 is formed on control region 3. A plurality ofpolycrystalline silicon diodes 25 are formed on insulation film 24.Diodes 25 are connected in series, thus forming a heat-sensitiveelement. Lateral type MOS transistor 26, polycrystalline siliconresistor 27 and constant-voltage zener diode 28 are also provided arounddiode 25. Transistor 26, resistor 27 and zener diode 28 form a controlsection.

The structure of the semiconductor device will be described in greaterdetail, with reference to FIG. 2. Semiconductor substrate 11 is an N⁺type silicon substrate 111. N₋ type silicon epitaxial layer 112 isformed on the substrate 111. Deep P type diffusion layer 291 is formedin that portion of epitaxial layer 112 which corresponds to power region12. P type diffusion layer 292, which corresponds to control region 13,is formed in a similar manner. Further, shallow P type layer 30, whichcorresponds to P type diffusion layer 291, is formed. Further, N⁺ typediffusion layer 31, which corresponds to vertical type MOS transistor22, is formed. N⁺ type diffusion layers 321 and 322, which correspond tolateral type MOS transistor 26, are formed. N⁺ type diffusion layer 33,which corresponds to constant-voltage zener diode 28, is formed.Further, P⁺ type diffusion layer 34 is formed.

Power MOS transistor 22 comprises silicon epitaxial layer 112, siliconsubstrate 111, and drain D made of drain electrode 35. Gate G oftransistor 22 is formed of polycrystalline silicon layer 37 formed ongate oxide film 36. Source S of transistor 22 is made of aluminumelectrode 39 covering inter-layer insulation film 38 which in turncovers up polycrystalline silicon layer 37.

When gate voltage is applied to gate G from terminal 40 through bondingpad 14, a channel is formed between silicon epitaxial layer 112 and N⁺type diffusion layers 31 and 32. As a result, a current flows betweensource S and drain D, more precisely between terminals 41 and 42.

As has been described, P type diffusion layer 291 is formed partly in Ptype diffusion layer 30, and is deeper than layer 30. Hence, power MOStransistor 22 can be protected against an excessively high voltage. Ptype diffusion layers 30 and 291 having this specific positionalrelation determine the breakdown voltage of MOS transistor 22.

Lateral type MOS transistor 26 has a source formed of N⁺ type diffusionlayer 321 and aluminum electrode 43, a drain made of N⁺ type diffusionlayer 322 aluminum electrode 44, and a gate made of polycrystallinesilicon layer 46. Silicon layer 46 is formed on gate oxide film 45 whichin turn is formed partly on layers 321 and 322 and partly on P typediffusion layer 292. When a gate voltage is applied to the gate fromterminal 47, an N channel is formed among polycrystalline silicon layer46 and N⁺ type diffusion layers 321 and 322. As a result, a currentflows between terminal 48 (source) and terminal 49 (drain).

Constant-voltage zener diode 28 comprises diffusion layers 33 and 34.Aluminum electrodes 50 and 51 contact layers 34 and 33, respectively.Electrodes 10 and 51 are connected to terminals 52 and 53.

Insulation film 24 of, for example, SiO₂ is formed by thermal oxidationon diffusion layer 292 occupying control region 13. Polycrystallinesilicon resistor 27 and polycrystalline silicon diode 25 having a PNjunction are formed on insulation film 24. Resistor 27 is made ofpolycrystalline silicon layer 55. Aluminum electrodes 56 and 57 contactlayer 55 and are connected to terminals 58 and 59. Diode 25 is formed bydiffusing a prescribed impurity into polycrystalline silicon layer 60,thereby forming a PN junction. Aluminum electrode 61 is formed on the Ptype portion of layer 60, and aluminum electrode 62 is formed on the Ntype portion of layer 60. Electrodes 61 and 62 are connected toterminals 63 and 64, respectively.

FIG. 3 is an equivalent circuit diagram showing the semiconductor deviceof FIG. 2. In this figure, the same numerals are used to designate thesame components as shown in FIGS. 1 and 2, except that numerals 271 to273 denote polycrystalline silicon resistors, R_(L) represents a loadresistor connected to an external device, and Vdd denotes an externalpower source.

When the temperature of silicon substrate 111 is below a predeterminedvalue, that is, when the junction of power MOS transistor 22 is atnormal temperature, transistor 22 is turned on by input voltage Vin.When the temperature of silicon substrate 111 rises above thepredetermined value, that is, when the junction of power MOS transistor22 is at excessively high temperature, however, the forward voltage ofpolycrystalline silicon diode 25, which functions as a heat-sensitiveelement, lowers. Diode 25 has a particular negative temperaturecoefficient. Hence, the more it is heated, the more its forward voltagefalls. When the forward voltage of diode 25 lowers, resistor 273 raisesthe gate-source voltage of lateral MOS transistor 26.

When its gate-source voltage rises, M0S transistor 26 is turned on. Ifresistor 272 has a resistance much higher than the resistance which MOStransistor 26 has while transistor 26 is on, the potential at point 40where the drain electrode of transistor 26 is located, and where gate Gof power MOS transistor 22 is coupled, will abruptly fail when thetemperature of substrate 111 rises above the predetermined value.

FIG. 4 shows the relationship between the gate voltage Vg and drainvoltage Vd of power MOS transistor 22, on the one hand, and thetemperature of the junction of this MOS transistor on the other. Whenthe temperature of the junction rises to 130° C. or thereabouts,transistor 26 is turned on thereby quickly reducing gate voltage Vg tozero volt to protect power MOS transistor 22. To be precise, when thistemperature rises to about 130° C., transistor 22 is forcibly turnedoff, thereby protecting the elements from breakdown.

As has been stated, polycrystalline silicon diode 25 (i.e., theheat-sensing element) and silicon resistor 27 are provided on insulationfilm 24 formed on control region 13. This structural feature makes iteasy to trim these elements independently. Further, due to this feature,no parasitic actions occur between these elements. Moreover, sincepolycrystalline silicon diode 25, which functions as a heat-sensitiveelement, is located in control region 13, i.e., the center portion ofsubstrate 11, it can accurately measure the temperature rise ofsubstrate 11 resulting from a junction temperature rise of power MOStransistor 22, and can therefore ensure a reliable protection of theelements from breakdown.

The elements on insulation film 24 can be made in the same steps aspower MOS transistor 22 In other words, no additional steps need to becarried out to manufacture these elements. Polycrystalline siliconresistor also 27 can be trimmed. Hence, its resistance can be accuratelyadjusted to any reference temperature selected, after the semiconductordevice has been manufactured. Alternatively, the reference temperaturecan be set to any desired value by forming a required number of PNjunctions in polycrystalline silicon diode 27.

In the above embodiment, silicon diode 25 and silicon resistor 27, bothformed in control region 13, are formed on insulation film 24. All theother elements can also be formed on insulation film 24. As is shown inFIG. 5, for example, it is possible to form lateral MOS transistor 261on insulation film 24 and constant-voltage zener diode 28 directly ondiffusion layer 292. Alternatively, lateral transistor 261 can be formedon diffusion layer 292, and zener diode 28 can be formed on insulationfilm 24.

In the above embodiment, MOS transistors 22 and 26 have an N channel.Needless to say, they can be so formed as to have a P channel. Thesemiconductor device of this embodiment has an active element, that is,a power MOS transistor. This MOS transistor can be replaced by a bipolartransistor or a power IC. Further, polycrystalline silicon diode 25,which functions as the heat-sensitive element, can be replaced by athermistor of the ordinary type. Moreover, the polycrystalline siliconresistors can be replaced by resistors made of tantalum nitride.

Generally, it is difficult to manufacture diodes having a designbreakdown characteristic or a design temperature characteristics whenthe diodes are made of polycrystalline silicon formed by depositingsilicon. The polycrystalline silicon diode used in the present inventionhas both a design breakdown characteristic and a design temperaturecharacteristic. It comprises a P type region and an N type region whichform a PN junction. Furthermore, the low impurity-concentration portionsof the N type and P type regions have impurity concentrations of 1×10¹⁹cm⁻³ or more.

In the polycrystalline silicon diode, that region of a silicon island,which will become an N type region, is doped with phosphorus, an N typeimpurity, and that region, which will become a P type region, is dopedwith boron, a P type impurity. The concentration of boron influencesvery much the breakdown characteristic and temperature characteristic ofthe diode. The concentration of boron is selected to be 1×10¹⁹ cm⁻³,thus imparting the desired breakdown characteristic and temperaturecharacteristic to the diode. Hence, the diode is greatly reliable.

With reference to FIG. 2, the process of forming polycrystalline diode25 will be briefly explained.

First, a polycrystalline silicon film having a thickness of about 2000 Åto about 5000 Å is formed by the CVD method on insulation film 24 whichhas been formed by thermal oxidation of silicon substrate 11. Thesilicon film is then patterned by plasma etching. Phosphorus ions areinjected into the patterned polycrystalline silicon film, therebyforming an N type region. Further, boron ions are injected into thatportion of the silicon film which surrounds the N type region, thusforming a P type region. Thereafter, the unfinished product is heated,thus activating the phosphorus and boron ions. An inter-layer insulationfilm of oxide silicon or the like is then formed by the CVD method onthe upper surface of the unfinished product.

The polycrystalline silicon used for forming the diode of the structuredescribed above is made up of countless crystals. Trap levels exist atthe interfaces among these crystals. Carriers are therefore trapped inthe trap level, inevitably building up a barrier potential.

This barrier potential largely depends on the quality of thepolycrystalline silicon film. It will ultimately much influence thecharacteristics of the polycrystalline silicon diode. More specifically,the value of barrier potential is determined by the amount of theimpurity contained in the polycrystalline silicon. The greater thecontent of the impurity, the lower the barrier potential. Therefore, thelow impurity-concentration portions of the N and P type regions formingthe polycrystalline silicon diode must have a relatively high impurityconcentration in order to stabilize the characteristics of the diode.

FIG. 6 represents the relationship between the boron concentration ofthe P type region of the diode and the breakdown voltage of the diode.FIG. 7 shows the relationship between this boron concentration and thedifference in breakdown voltage among the diodes. FIG. 8 illustrates therelationship between the boron concentration and the temperaturecoefficient of forward voltage. FIG. 9 shows the relationship betweenthe boron concentration and the difference in temperature coefficientamong the diodes.

As is shown in FIGS. 7 and 9, when the P type region has a boronconcentration of 1×10¹⁹ cm⁻³ or more, the difference of breakdownvoltage and the difference of the temperature coefficient sharplydecrease. Polycrystalline silicon diode 25, whose P type region has aboron concentration of 1×10¹⁹ cm⁻³, has both a design withstand voltageand a design temperature characteristic.

Instead of controlling the boron concentration of the P type region ofdiode 25, the concentration of an N type impurity (e.g., phosphorus orarsenic) can be controlled for the purpose of imparting design withstandvoltage and temperature characteristics to the diode. In this case, too,the concentration of the N type impurity is set to be 1×10¹⁹ cm⁻³.

FIG. 10 shows another control section which is formed in control region13. This control section comprises unit 100 for protecting the deviceagainst an excessive temperature rise, hysteresis unit 200 andgate-protecting unit 300. Unit 100 is similar to the protective unitshown in FIG. 3.

To fabricate the control section of FIG. 10, polycrystalline silicondiode 25, lateral type MOS transistor 26, polycrystalline siliconresistor 27 (271-275), constant-voltage zener diode 28, and the like areformed on control region 13, i.e., the center portion of semiconductorsubstrate 11, side by side in the same plane as is illustrated inFIG. 1. Polycrystalline silicon diode 25, or a heat-sensitive element,is thus composed of a plurality of silicon islands. The silicon islandsform the PN junctions of series-connected diode elements.

Hysteresis unit 200 comprises MOS transistor 70, diode 71, etc. whichare formed in control region 13. MOS transistor 70 is a lateral typetransistor. Unit 300 also comprises a number of diodes 73 which areformed either within or without control region 13.

The control section of FIG. 10 is advantageous over the control sectionof FIG. 3 in the following respect. In the section shown in FIG. 3, thedrain of power MOS transistor 22 may oscillate when it is heated to thereference temperature or thereabouts. In the control section of FIG. 10,hysteresis unit 200 gives hysteresis to the operating point of controlMOS transistor 26, thereby preventing the drain of power MOS transistorfrom oscillating. Hence, lateral type MOS transistor 70, resistor 275and level-shifting diode 71 cooperate to vary the potential at the pointcorresponding to the gate of lateral type MOS transistor 26. Gatevoltage Vg of power MOS transistor 22 can thereby have such hysteresisas is illustrated in FIG. 11.

The operation of the control section shown in FIG. 10 now will bedescribed in detail.

As long as the temperature of silicon substrate 111 remains below thereference value, power MOS transistor 22 is on due to input voltage Vin,and MOS transistor 70 is also on. The resistance which MOS transistor 70has in this condition is negligibly small in comparison with theresistance of resistor 275. Therefore, the gate voltage of MOStransistor 26 is determined by the resistance of resistor 273 and thecurrent flowing through resistor 273.

When the temperature of silicon substrate 111 rises above the referencevalue, the forward voltage of polycrystalline silicon diode 25 (i.e.,the heat-sensitive element) falls. As a result, the voltage between theends of resistor 273 proportionately rises. When this voltage exceeds apredetermined value, MOS transistor 26 is turned on. More specifically,MOS transistor 26 is turned on when its junction temperature rises to150° C. or thereabouts. Therefore, the gate voltage Vg of power MOStransistor 22 falls, and transistor 22 is turned off. Also, lateral typeMOS transistor 70 is turned off. While power MOS transistor 22 is off,resistor 275 and diode 71 are connected in series to resistor 273. Theresistance of this portion therefore increases. As a result, the voltageapplied to the gate of MOS transistor 26 rises, and the operationtemperature falls a little (to 120° C.) as shown in FIG. 11.Consequently, the voltage drop in group of diodes 25 increases, wherebyMOS transistor 26 is turned off. That is, sufficient hysteresis isprovided, and the oscillation of power MOS transistor 22 is prevented.In other words, power MOS transistor 22 has such hysteresis that it isturned off when its junction temperature rises above 150° C., and isturned on when its junction temperature falls below 120° C.

FIG. 12 shows still another control section according to the invention.This control section is, so to speak, a combination of the controlsection shown in FIG. 10 and unit 400 for protecting the semiconductordevice against an excessively large current. Unit 400 comprises verticaltype power MOS transistor 74.

Transistor 74 is formed in the region where power MOS transistor 22 isformed. Its source electrode occupies a tiny portion of this region,i.e., 1/100 to 1/3000 of the region, and is electrically separated fromtransistor 22. Transistor 74 is driven by the same gate voltage as isapplied to power MOS transistor 22, and it outputs a small currentproportional to the output current of power MOS transistor 22. MOStransistor 75 is formed in the same substrate as other lateral MOStransistors 26 and 70.

When external load R_(L) is short-circuited, an excessively largecurrent flows through power MOS transistor 22. Then, an excessivelylarge current also flows into MOS transistor 74 which functions as acurrent-detecting element, thereby raising the gate potential of MOStransistor 75. When this gate potential reaches a threshold voltage ofMOS transistor 75, a current will flow into transistor 75. As a result,the gate voltage of power MOS transistor 22 falls, and the outputcurrent of transistor 22 increases.

Unit 400 has a stabilization point which is determined by circuitconstants, such as the area ratio of power MOS transistor 22 to MOStransistor 74, the resistance of resistor 276, the threshold voltage ofMOS transistor 75, and the ratio of the on-resistance of transistor 74to the resistance of resistor 276. Hence, a maximum current isdetermined in accordance with this stabilization point.

Without unit 400, the current ability of power MOS transistor 22 wouldindefinitely increases as the drain voltage increases. As long as thedrain voltage is below a specific value (e.g., 2 volts), the currentability of transistor 22 increases in the same way as it would if unit400 were not provided. However, once the drain voltage rises above thisspecific value, the drain current of MOS transistor 22 remains at asubstantially constant value.

Hence, even if the external load, for example, is short-circuited, theload current is limited to a specific value. The semiconductor elementsand wiring of the semiconductor device thereby can be reliably protectedfrom an excessively large current.

What is claimed is:
 1. A semiconductor device comprising:a semiconductorbody; heat generating means, including an active semiconductor element;an insulation film formed on a surface of said semiconductor body; aheat-sensitive element including at least one diode element formed by aPN junction being formed in a polycrystalline silicon semiconductorlayer on said insulation film in such a manner that said heat-sensitiveelement is separated from said semiconductor body, said heat-sensitiveelement detecting the temperature of said semiconductor body varying dueto the heat generated by said heat generating means; and means,electrically connected to said heat-sensitive element, for deriving, asa temperature detection signal, the level of a forward voltage dropgenerate din said at least one diode element.
 2. A semiconductor deviceaccording to claim 1, wherein said diode element having said PN junctionincludes a pair of highly doped region and lowly doped region, theimpurity concentration of said lowly doped region being 1×10¹⁹ cm⁻³ ormore.
 3. A semiconductor device according to claim 1, wherein said heatgenerating means is an active semiconductor element formed in saidsemiconductor body.
 4. A semiconductor device according to claim 3,wherein said active semiconductor element is a vertical type power MOStransistor element.
 5. A semiconductor device according to claim 1,wherein said diode element having said PN junction includes a pair ofhighly doped region and lowly doped region, the impurity concentrationof said lowly doped region being 1×10¹⁹ cm⁻³ or more; and said heatgenerating means is an active semiconductor element formed in saidsemiconductor body, said active semiconductor element being a verticaltype power MOS transistor.
 6. A semiconductor device according to claim3, wherein said heat-sensitive element has a plurality of PN junctionsincluding each of a pair of highly doped regions and lowly dopedregions, and further includes a conductive member which connects saidplurality of PN junctions to each other in series in the same polardirection so as to form a plurality of diode elements, the impurityconcentration of said lowly doped region being 1×10¹⁹ cm⁻³ or more; andthe temperature of said semiconductor body generated by said activesemiconductor element is detected by said plurality of diodes.
 7. Asemiconductor device according to claim 1, wherein said heat-sensitiveelement is located in the substantially center part of saidsemiconductor body, said heat generating means surrounding saidheat-sensitive element region.